1. Field of the Invention
The present invention relates to an organic EL display device, and more particularly, to a top emission type organic EL display device which has a high and uniform brightness in its screen.
2. Background Art
Organic EL display device is classified into a bottom emission type which takes out light emitted from an organic EL layer in the direction of glass substrate on which the organic EL layer and the like are formed and a top emission type which takes out the light in the direction opposite to the glass substrate on which the organic EL layer and the like are formed. The top emission type has an advantage that the respective organic EL layers can ensure a large area, thus increasing the brightness of a display.
In the organic EL display device, an organic EL layer is sandwiched between a lower electrode and an upper electrode, and light emission from the organic EL layer is controlled by applying a predetermined voltage to the upper electrode and applying a data signal voltage to the lower electrode, thereby forming an image. The data signal voltage is supplied to the lower electrode through a thin film transistor (TFT). In the top emission type organic EL display device, an organic EL layer can be formed on the TFT, thereby enabling a light emission area to increase.
In the organic EL display device, current needs to be supplied from the upper electrode to the organic EL layer. The top emission type organic EL display device has to take out light through the upper electrode, and hence, the upper electrode must be transparent. As a transparent electrode, an ITO (Indium Tin oxide) film, an InZnO (Indium Zinc Oxide) film, a ZnO (zinc oxide) film, and a SnO (tin oxide) film, or the like, is used; however, these metallic oxide conductive films exhibit large resistance. When a film thickness of the upper electrode is increased in order to reduce the resistance in the upper electrode, there arise the following problems.
As one problem, getting the upper electrode thicker causes absorption of light from an organic EL layer, hence to deteriorate brightness. As the other problem, when the upper electrode gets thicker, there is a phenomenon that a specified wavelength light is different to take out owing to the interference of light within the upper electrode.
JP-A-2007-328141 discloses a structure in which light transmittance is enhanced in a liquid crystal display device by varying a film thickness of an opposite electrode formed on an opposite substrate set opposite to the TFT substrate on which TFT and pixels are formed, for every color.